I see Michael is on this, but here is my two cents. I realize others may not agree, whatev.
A crackle finish is done with a size or medium being applied to a surface and then one coat of paint applied to it,a flat paint. Maybe it has a top coat on it,maybe not. Having said that,crackle finishes on walls are not usually very thick.
If it were my client, I would prime, not necessarily with oil and then spot skim only where necessary. It's a foyer, it seems a bit overkill to skim up near a 17-23 ft ceiling line,hmmm? I would only skim anything objectionable within reason, such as a couple feet above eye level and down, or perhaps along the stair railing where people will see it.
I love biscuits with lots of butter,makes it go down easier.
